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Momentum(?) problem

  1. Nov 15, 2006 #1
    Two blocks are free to slide along a frictionless wooden track ABC as shown in Figure P9.20. The block of mass m1 = 4.93 kg is released from A. Protruding from its front end is the north pole of a strong magnet, repelling the north pole of an identical magnet embedded in the back end of the block of mass m2 = 9.60 kg, initially at rest. The two blocks never touch. Calculate the maximum height to which m1 rises after the elastic collision.

    The figure shows m1 on a curved ramp at a height of 5 m.

    Since it is elastic, I know energy and momentum are conserved. So I have:

    (1/2)m1*v1o^2+(1/2)m2*v2o^2 = (1/2)m1*v1f^2+(1/2)m2*v2f^2
    m1*v1o+m2*v2o = m1*v1f+m2*v2f

    m2 is initially at rest, so v2o=0. Now I am not sure how I am supposed to use these to find height, or anything at all for that matter. Can anyone give me a point in the right direction?
  2. jcsd
  3. Nov 15, 2006 #2

    Doc Al

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    Staff: Mentor

    Two equations, two unknowns. One of those unknowns will give you the energy of m1 immediately following the collision. Mechanical energy is conserved.
  4. Nov 15, 2006 #3
    Well, mechanical energy is KE+PE, so

    (1/2)m1*v1f+m1*g*hf = (1/2)m1*v1o+m1*g*ho

    But I am not sure how to connect that to the other equations I have.

    I am having a hard time getting my head around this one.
  5. Nov 15, 2006 #4

    Doc Al

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    Staff: Mentor

    Since you didn't include the figure, I'm just guessing as to what it shows.

    Make sure you square those speeds in the KE terms. I assume v1o is the initial speed of m1 immediately after the collision. (That speed is called v1f in your collision equations.)

    The collision equations will give you the speed of m1 after the collision.

    There are three steps to this problem:
    (1) The fall of m1 from point A to where it collides with m2
    (2) The collision
    (3) The rise of m1​

    In each step, energy is conserved. In step 2, momentum is also conserved.
  6. Nov 15, 2006 #5
    Okay, I got it, thanks a lot. When you put it into three steps, something clicked and everything came out. Thanks again.
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